1. Field of the Invention
The present invention relates to a thin film multi-layer wiring substrate and, in particular, to a thin film multi-layer wiring substrate adapted for high frequency signal transmission, and to a method for manufacturing the same. The present invention also relates to a printed circuit board favorable for use in high frequency circuits.
2. Description of the Related Art
In the field of electronic machinery, high density mounting is advancing, and the signal speed of electronic machines is accelerating. Under such circumstances, noise reduction measures in signal transmission are becoming increasingly important.
As one type of noise reduction measure, formation of coaxial wiring in a thick film multi-layer printed circuit board is performed (Unexamined Patent Publication (Kokai) No. 4-267586).
In this coaxial wiring pattern formation method, after forming a lower conductive pattern by means of a thick film printing method or the like on the surface of a base substrate of alumina ceramics or the like, a light sensitive insulation film is formed thereon. After drying this insulation film, a via hole is formed in the insulation film by photolithography, and a conductive paste is introduced into the via hole and sintered to form a lateral conductor, whereafter a signal wiring pattern is formed on the insulation film by a thin film plating method. Then, by the same process, an insulating film, a via hole, and, by means of a conductive paste, a lateral conductor, are formed, whereafter an upper conductive pattern is formed to form the coaxial wiring structure.
In the method disclosed in Unexamined Patent Publication (Kokai) No. 4-267586, the gap between the signal wiring formed by the thick film printing method and the lateral conductor via hole formed by photolithography is set at approximately 100 micrometers in consideration of relative dislocation. This is because dislocation occurs when matching the via hole film to the previously formed conductive pattern, the cause of this dislocation being a film matching error, or alternatively, changes in the dimensions of the substrate, changes in the film dimensions, or the like.
Where the gap between the signal wiring and the lateral conductor is set at 100 micrometers in this way, the signal wiring pitch is limited to approximately 500 micrometers, it being problematic to set it any finer.
On the other hand, where transmitting a high frequency signal on a printed circuit board, it is easy for crosstalk to occur between signal line conductors, therefore various means for reducing this problem have been proposed.
To avoid crosstalk, electrical couplings between signal line conductors can be arranged sparingly; for example, a method of widening the gaps between signal line conductors, providing ground conductors in the vicinity of the signal line conductors, or the like is known.
As a printed circuit board structure based on the above method, using a strip line structure or micro strip line structure, sandwiching the circuit pattern layer of the signal line conductors between ground line pattern layers, is known, whereby the desired characteristic impedance can be realized and crosstalk reduced.
Also, the propagation characteristic of high frequency signals relies on the dielectric constant of the printed circuit board material and, therefore, in realizing a signal line conductor with little propagation loss, it is well known that a material with a low dielectric constant must be used.
In order to avoid crosstalk, where the method of widening the spaces between the signal line conductors is employed, this leads to the problem of reduced wiring density or signal transmission loss, and where the method of providing ground conductors between the signal line conductors is employed, although crosstalk between signal line conductors propagating the same signal is alleviated, prevention of crosstalk between signal line conductors propagating different signals is basically impossible.
Also, because there is no shield layer on the printed circuit board itself, it easily succumbs to the effect of extraneous noise and, in order to eliminate that effect, it is necessary to provide a conductive shielding case or the like to shield the printed circuit board.
Further, in recent years, in high frequency devices with clock frequencies in the order of gigahertz, although printed circuit board materials with dielectric constants in the order of 3, such as polyimide and the like, have been used, these printed circuit board materials are limited when it comes to further reducing the dielectric constant.